In some applications of scanned charged-particle beams, e.g., the use of scanned electron beams to sterilize materials, uniformity of charge deposition and a predictable beam energy are both important in order to achieve effective and efficient treatment of the material being irradiated. Loss of charge deposition or irradiation dose uniformity will occur if energy dispersion is uncorrected. Uncertainty in the depth of deposition will occur if beam energy is not monitored and controlled.
In the prior art, irradiation of material by an electron beam from a microwave electron linear accelerator, has been achieved by the use of a 90 degree bend magnet, in addition to a scanning dipole. U.S. Pat. No. 3,193,717 to Nunan, assigned in common with this patent, discloses apparatus for scanning a beam using a 90.degree. magnet followed by a scanning dipole. U.S Pat. No. 4,063,098 to H. A. Enge, discloses a quadrupole magnet after a scan magnet and a bending magnet and before the articles to be irradiated. The quadrupole magnet of the Enge patent compensates for the energy dispersion of scanned charged particles. This quadrupole magnet is asymmetric, with a relatively narrow gap between those poles through which the higher momentum particles pass, to compensate for the dispersion effect which occurs in the scanning process. A symmetric quadrupole would compensate for deflection dispersion only, but the asymmetric structure compensates both for the deflection and scanning dispersion. Both the Enge apparatus and the Nunan apparatus are bulky and expensive because they require separate bending, scanning and focussing devices.
Some scanners of the prior art used divergent scanned beams. If the irradiated subject is being moved across the divergent beam in the bend plane, an averaging takes place which eliminates adverse effects of the divergent beam. Where the irradiated subject is being moved across the beam in the direction transverse to the bend plane, the divergent beam causes problems of uneven dosage across the target and ineffecient use of the beam at the edges of the scan.